Conformal and hidden antennas are desirable on many mobile platforms for reasons of aerodynamics and styling, among others. Such antennas have been implemented as or on Artificial Impedance Surfaces (AIS) and have been associated with Frequency Selective Surfaces (FSS). AIS can also be referred to as Artificial Magnetic Conductors (AMC), particularly when a separate antenna is disposed on it. AMC, AIS and FSS are all well known in the art and look very similar to each other which means that persons skilled in the art have not always maintained bright lines of distinction between these terms. AMC, AIS and FSS are generically referred to as impedance surfaces and if they are tunable using active circuits (to generate negative capacitance or negative inductance for example) they are referred to as Active Artificial Impedance Surfaces herein.
AIS and AMC tend to have a ground plane which is closely spaced from an array of small, electrically conductive patches. The AIS can serve as an antenna itself whereas an AMC tends to have, in use, a separate antenna disposed on it. Other than the manner of use (and where an antenna is specifically mounted on one), an AIS and a AMC are otherwise basically pretty much identical. The FSS, on the other hand, tends to have no ground plane and therefor it can be opaque (reflective) at certain frequencies and transmissive at other frequencies, much like an optical filter. The FSS looks much like a AMC or an AIS, except that there is typically no ground plane as noted above. All of these devices (AMC, AIS and FSS) operate at RF frequencies and have many applications at UHF and higher frequencies. All of these devices (AMC, AIS and FSS) include two dimensional arrays of metallic patches spaced in a subwavelength periodic grid compared to the RF frequencies at which the devices are designed to operate. The metallic patches come in numerous possible geometric shapes.
At VHF and UHF frequencies, however, many relevant platforms which might use AIS/FSS antenna technology are on the order of one wavelength or less in size, which dictates that the antennas be electrically small. Therefore, the performance is limited by the fundamental bandwidth-efficiency tradeoff given by the Chu limit when passive matching is employed.
A wideband artificial magnetic conductor (AMC), a special case of an AIS, can be realized by loading a passive artificial magnetic conductor structure with NFCs (i.e. negative inductance and negative capacitance) as suggested by D. J. Kern, D. H. Werner and M. J. Wilhelm, “Active Negative Impedance Loaded EBG Structures for the Realization of Ultra-Wideband Artificial Magnetic Conductor”, in Proc. IEEE Antennas and Propagation Society Int. Symp., 2003, pp 427-430. Only simulation results were presented in this paper with ideal NFCs; no details are provided of how to realize the stable NFCs needed in such an application.
NFCs (non-foster circuits) are so named because they violate Foster's reactance theorem and overcome these limitations by canceling the antenna or surface immittance over broad bandwidths with negative inductors or negative capacitors. See the article by Kern mentioned above and also S. E. Sussman-Fort and R. M, Rudish, “Non-Foster impedance matching of electrically-small antennas, “IEEE Trans. Antennas and Propagat.”, vol. 57, no, 8, August 2009. These non-passive reactive elements are synthesized using Negative Impedance Converters (NICs) or Negative Impedance Inverters (NIIs). NICs are feedback circuits that convert a passive capacitor to a negative capacitor while NIIs are feedback circuits which convert a passive capacitor to a negative inductor. It is also possible to use passive inductors to make negative capacitors and negative inductors using these circuits, but since a passive capacitor is easier to make using semiconductor fabrication techniques, it is assumed herein that a passive capacitor is preferably used to generate a negative inductance (using a NII) or a negative capacitance (using a NIC) as needed herein.
The main challenge in realizing NFCs is stability; NICs and NIIs are conditionally stable, and the stability margin typically approaches zero as immittance cancellation becomes more complete. For this reason, few stable demonstrations are reported in the literature at and above VHF frequencies. Sussman-Fort and Rudish noted above and K. Song and R. G. Rojas, “Non-Foster impedance matching of electrically small antennas,” Proc. IEEE Ant. Prop. Int. Symp., Jul. 2010 have reported negative-capacitance circuits and measured improvement in the realized gain of electrically small monopole antennas.
A well-known class of AIS consists of printed metallic patterns on an electrically thin, grounded dielectric substrate. They can be used to synthesize narrow-band Artificial Magnetic Conductors (AMC) for the realization of low profile antennas as well as suppress surface waves over a narrow bandwidth. They can be made tunable. See, for example, U.S. Pat. No. 6,538,621 to Sievenpiper et al mentioned above. Furthermore, HRL Laboratories of Malibu, Calif. has shown that they can be used to build directional antennas with arbitrary radiation patterns and direct incident energy around obstacles using conformal surfaces with a holographic patterning technique. See the paper noted above by B. H. Fong, et al. entitled “Scalar and Tensor Holographic Artificial Impedance Surfaces”. One issue with the use of NFCs in these arrays is that the power and control wiring to the NFCs can affect the electromagnetic properties of an active AIS system. Furthermore, it can be challenging to run the wires over a distance more than about an inch. Most importantly, this approach does not extend to bulk metamaterials or metasurfaces with no ground plane.
Power to the NFCs can be provided by batteries: S. D. Stearns, “Non-Foster circuits and stability theory,” in proceedings, 2011 IEEE Antennas and Propagation Intl. Symposium, pp. 1942-1945. However, batteries are large so that integration into smaller areas such as 1 square millimeter is not practical. In addition, batteries are heavy and are not practical in extreme temperatures and in high shock applications. More importantly, a battery powered NFC cannot be controlled remotely, either to turn it on/off or to vary the circuit parameters. Furthermore, it would be undesirable for an operator to control NFCs manually in an array/AIS environment which may have hundreds of NFCs.
In A. Adonin, et al. “Monolith Optoelectronic Integrated Circuit With Built-In Photo-voltaic Supply For Control and Monitoring,” 1998 IEEE International conference on electronics, circuits and systems, vol. 2, pp. 529-531, a low power IC is powered by an integrated PV cell network. This is a low power digital circuit, not an RF circuit. The goal seems to be that it is powered by ambient light.
Schaffner, James H. and Jones, Dennis C., “Single fiber optical links for simultaneous data and power transmission,” U.S. Pat. No. 7,941,022, May 10, 2011 describes how to use double core fiber to send power on one optical wavelength and data on another optical wavelength to a remote receiver. In this invention, the use of single mode double core fiber was necessary because of the long length of fiber needed for the application described in the patent. In the present invention, the fiber length is much shorter and the data rates needed for logic control (in some embodiments) is low enough that the double core, single mode fiber is not needed.